This invention relates to a xerographic process and apparatus useful in electronic reprographic, orthographic color imaging system, that is imaging systems capable of creating highlight color, graphics and data plots with color coding.
The invention can be utilized in the art of xerography or in the printing arts. In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a photoreceptor. The photoreceptor comprises a charge retentive surface. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation. The areas of charge dissipated on the photoreceptor correspond to residual or background voltage levels. Thus, the photoreceptor contains two voltage levels in the case of a binary digital system. In the case of a light/lens system a whole array of voltage levels are present on the photoreceptor.
This latent charge pattern is rendered visible by developing it with toner. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction.
The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
Conventional xerographic imaging techniques which were directed to monochrome image formation have been extended to the creation of color images including highlight color images. In one method of highlight color imaging, the images are created using a raster output scanner to form tri-level images.
The concept of tri-level, highlight color xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development systems are biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In highlight color xerography as taught by Gundlach, the xerographic contrast on the charge retentive surface or photoreceptor is divided into three levels, rather than two levels as in the case in conventional xerography. The photoreceptor is charged, typically to -900 volts. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged-area development, i.e. CAD) stays at the full photoreceptor potential (V.sub.CAD or V.sub.ddp). V.sub.ddp is the voltage on the photoreceptor due to the loss of voltage (otherwise known as dark decay) while the photoreceptor remains charged in the absence of light. The other image is exposed to discharge the photoreceptor to its residual potential, i.e. V.sub.DAD or Vc (typically -100 volts) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD) and the background area is exposed such as to reduce the photoreceptor potential to halfway between the V.sub.CAD and V.sub.DAD potentials, (typically -500 volts) and is referred to as V.sub.white or V.sub.w. The CAD developer is typically biased about 100 volts closer to V.sub.CAD than Vwhite (about -600 volts), and the DAD developer system is biased about -100 volts closer to V.sub.DAD than V.sub.white (about 400 volts). As will be appreciated, the highlight color need not be a different color but may have other distinguishing characteristics. For, example, one toner may be magnetic and the other non-magnetic.
As noted above, in conventional xerography the photoreceptor contains two voltage levels whereas in tri-level xerography three voltage levels are present. Thus, with tri-level imaging the image contrast voltage is substantially reduced from that of conventional xerography. This represents a significant limitation to extending the concept of tri-level imaging to full gamut color imaging. This is because the already small, relative to conventional xerography, development field for creation of the color image must be further reduced for each color to be created.
Multiple level imaging with a ROS such as utilized in tri-level xerography, as will be appreciated, is highly desirable because of the perfect image registration that is provided thereby.
Notwithstanding the limitation of reduced contrast images, attempts have made at extending the color gamut using tri-level xerography. For example, In U.S. Pat. No. 4,731,634 granted to Stark on Nov. 3, 1986 discloses a method and apparatus for printing images in black and at least one highlight color in a single pass of the imaging surface through the processing stations of the imaging apparatus. To this end, the exposure device which is a Raster Output Scanner (ROS) is operated to form four voltage levels comprising three image levels and a background level. An even lower contrast voltage is available for creating the additional highlight color image.